The present invention relates to a DC-offset canceller that removes DC components generated in a data receiver through digital processing. Particularly, the present invention relates to a DC-offset canceller capable of removing the DC components in a case where a frequency offset is included in the received signals having a training signal.
One of the conventional examples of the DC-offset canceller for compensating the DC-offset to be used in the data receiver is one disclosed in Japanese Patent Laid-Open No. 142248 (1990). In FIG. 7, the prime constituents of the DC-canceller disclosed in such an official gazette are illustrated as a block diagram.
According to the configuration shown in FIG. 7, a center frequency can be converted into a signal of frequency f2 as shown in FIG. 8(a) by detecting an input signal of frequency f1 supplied to an input terminal 201 by a detector 203 using a local oscillation frequency f1+f2) of a local oscillator 203. Then DC components are removed by a capacitor 204. The remainder of the signal is converted into a digital signal by an AD (analog to digital) converter 205, and then it is subjected to a frequency conversion using a sine wave of local oscillation frequency f2 generated from a local oscillator 206 through a frequency converter 207, resulting in an output of center frequency zero (0) as shown in FIG. 8(b).
In the case of the conventional DC-offset canceller described above, the DC component is removed by using the capacitor 204 after converting the frequency of input signal into a frequency which does not overlap DC component generated on an analog portion upstream to the capacitor 204. Thus, the capacitor 204 has no effect on an input signal component. A center frequency of the input signal eventually becomes zero (0) by means of the digital processing, so that there is no possibility of introduction the DC component. Therefore, a demodulation of the input signal can be performed without an affection due to the DC components generated on the analog portion.
The conventional DC-offset canceller as described above requires a wide range of frequency band for performing AD conversion on signals in varied frequencies.
The reason is that at least 2(f2+f3) of sampling frequency is required because frequency components of signals supplied to the AD converter 205 distribute to f2+f3 as shown in FIG. 8(a). If the electric circuit is of without the need for the DC-offset canceller, the sampling frequency may be 2f3 or more because frequency components of signals to be supplied to the AD converter 206 distribute to f3 as shown in FIG. 8(b).
If the sampling frequency is increased, the power consumption is also increased in proportion to the frequency increase in addition to an increase in the price of the AD converter itself. Therefore, there is a problem of the difficulty in keeping with trends to lower power consumption for the application of DC-offset canceller in any fields including personal digital assistants (PDAs).
In addition, the conventional DC-offset canceller performs the frequency conversion on a digital operation part, so that there is the need for generating a sine wave and performing a filter operation. For generating the sine wave on the digital processing, the DC-offset canceller generally uses a mode of pre-writing the value of sine wave on ROM. In this case, however, there is the problem that the capacity of ROM is increased depending on the conversion frequency.
Besides, there is another problem that the scale of the operation is increased because of the need for performing a multiplication several times in the filter operation.
Therefore, the present invention is accomplished in view of the problems of the prior art described above, so that it is an object of the present invention is to provide a DC-offset canceller which can be realized by a simple circuit with low power consumption.
According to an aspect of the present invention, there is provided a DC-offset canceller in a receiver of a communication system using a burst signal including a training sequence with a predetermined periodicity at the head thereof, comprising:
a quadrature demodulator for converting the received burst signal to a base band signal;
an AD converter for converting an output signal of the quadrature demodulator to a digital signal;
a one-cycle delay element for making a delay of an output signal of the AD converter corresponding to one-cycle of the training sequence;
a DC-offset detector for detecting a DC-offset component in the converted signal by the AD converter on the basis of an output signal of the AD converter and an output signal of the one-cycle delay element; and
a subtractor for removing the DC-offset component detected by the DC-offset detector from the output signal of the AD converter.
According to another aspect of the present invention, there is provided a DC-offset canceller in a receiver of a communication system that performs communication using a burst signal including a training sequence with a predetermined periodicity at the head thereof, comprising:
a quadrature demodulator for converting the received signal to a base band signal;
an AD converter for converting an output signal of the quadrature demodulator to a digital signal;
a one-cycle delay element for making a delay of an output signal of the AD converter corresponding to one-cycle of the training sequence;
a two-cycle delay element for making a delay of an output signal of the AD converter corresponding to two-cycle of the training sequence;
a DC-offset detector for detecting a DC-offset component in the converted signal by the AD converter on the basis of an output signal of the AD converter, an output signal of the one-cycle delay element, and an output signal of the two-cycle delay element; and
a subtractor for removing the DC-offset component detected by the DC-offset detector from the output signal of the AD converter.
According to other aspect of the present invention, there is provided a method for performing a DC-offset cancellation in a communication system that performs communication using a burst signal including a training sequence with a predetermined periodicity at the head thereof, comprising:
a first step for converting the received burst signal to a base band signal and then converting the base band signal to a digital signal;
a second step for making a delay of an output signal obtained in the first step so that the delay corresponds to one-cycle of the training sequence;
a third step for detecting a DC-offset component in the digital signal obtained in the first step from both of the digital signal obtained in the first step and the digital signal obtained in the second step; and
a fourth step for removing the DC-offset component obtained in the third step from the digital signal obtained in the first step.
According to still other aspect of the present invention, there is provided a method for performing a DC-offset cancellation in a communication system that performs communication using a burst signal including a training sequence with a predetermined periodicity at the head thereof, comprising:
a first step for converting the received burst signal to a base band signal and then converting the base band signal to a digital signal;
a second step for making a delay of an output signal obtained in the first step so that the delay corresponds to one-cycle of the training sequence;
a third step for making a delay of an output signal of the second means corresponding to two cycle of the training sequence;
a fourth step for detecting a DC-offset component in the digital signal obtained in the first step from the digital signals obtained in the first to third steps; and
a fifth step for removing the DC-offset component obtained in the fourth step from the digital signal obtained in the first step.
According to still further aspect of the present invention, there is provided a DC-offset canceller comprising:
a first means for converting a received burst signal including a training sequence with a predetermined periodicity to a base band signal;
a second means for converting an output signal of the first means to a digital signal;
a third means for making a delay of an output signal of the second means corresponding to one-cycle of the training sequence;
a fourth means for detecting a DC-offset component in the converted signal by the second means on the basis of output signals of the second and the third means; and
a fifth means for removing the DC-offset component detected by the fourth means from the output signal of the second means.
According to other aspect of the present invention, there is provided a DC-offset canceller comprising:
a first means for converting a received burst signal including a training sequence with a predetermined periodicity to a base band signal;
a second means for converting an output signal of the first means to a digital signal;
a third means for making a delay of an output signal of the second means corresponding to one-cycle of the training sequence;
a fourth means for making a delay of an output signal of the second means corresponding to two cycle of the training sequence;
a fifth means for detecting a DC-offset component in the converted signal by the second means on the basis of output signals of the second, third and fourth means; and
a sixth means for removing the DC-offset component detected by the fifth means from the output signal of the second means.
In a burst signal transmission, the present invention cancels a DC-offset component by adding a training sequence on the top of burst, detecting the DC-offset component by the use of only a digital operation during receiving the training sequence on the part of receiver, and canceling the DC-offset component from a series of signals in the whole burst on the basis of the detected value.
More specifically, the present invention uses a pattern with a predetermined periodicity as the training sequence added on the top of a burst signal. The DC-offset component is removed from the received signal by detecting the difference between the received signal of training sequence and the received signal one-cycle apart therefrom, which is proportional to the amount of the DC-offset.
Furthermore, the present invention also cancels a frequency offset in addition to the DC-offset since the frequency offset may be extracted from the received signals at three points, a received signal of training sequence, a received signal one-cycle apart therefrom, and a received signal two-cycle apart therefrom.
Other objects and features will be clarified from the following description with reference to attached drawings.